Where are the computer-controlled DACs, amplifiers and preamps?

[occamsrazor]

why can't I control the volume of the music I'm playing from the same computer I used to select the file that's playing?

 

Ideally you want a DAC with USB HID volume control. Some do, but not all....

 

http://www.computeraudiophile.com/f6-dac-digital-analog-conversion/digital-analogue-converter-pre-support-universal-serial-bus-industry-standard-cables-connectors-and-communications-protocols-between-computers-and-electronic-devices-hid-volume-control-27174/